This invention relates to a process for the preparation of aryl haloformates, such as phenyl chloroformate, and diaryl carbonates, such as diphenyl carbonate, from phenols and carbonyl halides, such as phosgene.
Aryl haloformates and diaryl carbonates are well known as monomers for the production of polycarbonate plastics.
Up to the present time the direct reaction between phenols and phosgene is so slow that the reaction has not been practiced commercially. It is feasible, however, to prepare aryl chloroformates and diaryl carbonates by reacting phenols with phosgene in the presence of a stoichiometric amount of sodium hydroxide. The base reacts with hydrogen chloride liberated in the process to form a stoichiometric amount of waste salt, which disadvantageously must be separated from the product mixture and disposed. More disadvantageously, in commercial applications the process is conducted in a two-phase system comprising water and an organic solvent which reduces the productivity of the process.
It is also known that phenols react with phosgene in the presence of a stoichiometric amount of tertiary amine to yield aryl chloroformates and diaryl carbonates. The tertiary amine reacts with hydrogen chloride liberated in the process to form a stoichiometric amount of amine hydrochloride salt. Disadvantageously, the amine hydrochloride must also be removed from the reaction mixture and disposed. Moreover, the use of tertiary amine requires the process to be conducted in an organic solvent. The waste treatment and use of organic solvent adversely affect the economics of the process.
U.S. Pat. No. 4,366,102 discloses a method of preparing aromatic chloroformic acid esters by reacting phenols and phosgene in a homogeneous liquid phase and in the presence of a catalyst consisting of an organic phosphorus compound. The phosphorus compound is disclosed to be an organic phosphine, phosphine oxide, phosphine halide, phosphonium salt, or halogenophosphine. These compounds are undesirable, because they are expensive. This patent also teaches at Example 3 a comparative experiment wherein phenol and phosgene are heated in the presence of triethyl phosphite to yield predominantly diphenyl carbonate and a minor amount of chloroformic acid phenyl ester.
It would be advantageous to have a process of preparing aryl haloformates and diaryl carbonates which does not require a stoichiometric amount of base, such as, sodium hydroxide or tertiary amine. Such a process would eliminate the need for separating and disposing of or treating a waste salt stream. It would also be desirable if the process did not require expensive catalysts, such as, organic phosphines.